This invention relates to an hydraulic actuator for a vehicle brake, and particularly for an internal shoe drum brake, the actuator incorporating an automatic adjuster operable, in response to excess brake-applying movement of a braking element, resulting for example from normal lining wear, to make a compensating adjustment for such excess movement.
An actuator has already been proposed, primarily for a shoe drum brake, in which the automatic adjuster is arranged to act between a hydraulic piston and a relatively fixed cylinder body of the actuator, the adjuster including a variable length strut formed by an adjuster shaft threadedly engaging the piston by way of a non-reversible screw thread, a portion of the shaft being provided with a reversible thread engaged by an annular body which is spring-urged towards engagement with a conical clutch face formed on the cylinder body. Outward movement of the piston for brake actuation causes the shaft to relieve the spring force urging the ring against its clutch face, thereby enabling the ring to slip against said face and be driven in rotation by the reversible thread relative to the shaft. Upon brake release, the piston is retracted by the shoe return springs and the ring is again urged against its clutch seat so that torque then generated in the reversible thread causes the shaft to rotate relative to the restrained ring, thereby lengthening the strut and setting a new retracted position of the shoe associated with the actuator.
The operation of this adjuster depends upon generating a greater torque in the non-reversible thread than that induced at the reversible thread in order to ensure that the shaft does not rotate with the ring and this is accomplished by providing a compression spring acting axially between the shaft and cylinder body.
It has been found that when the shoe return spring force across the adjuster is relieved as a result of brake actuation, vibrations arising in the brake assembly can cause the adjuster shaft and compression spring to oscillate with resultant fluctuations in load in the non-reversible thread. Because the compression spring is lengthened and its force thereby diminished during outward movement of the piston, the residual force which it exerts in this condition may not be sufficient to prevent random rotation of the adjuster shaft which can lead to unwanted variation in the adjustment condition of the adjuster and a consequent difference in pedal travel upon subsequent brake application.
A further disadvantage of the compression spring is that the load which it imposes on the non-reversible thread is at a maximum when the brakes are retracted and must be overcome to permit manual de-adjustment of the brake which may be required, for example, for servicing purposes.